Just used rpiboot to get an Ultibo kernel.bin working on a Zero with no SDcard.
Used the standard start.elf and bootload.bin plus kernel.bin in the msd folder.

USB takes a bit of time to connect, so it is not as fast as normal <2 sec SDcard boot.
But this now means a Cluster hat of 4 Zero's and no SDcards needed for the Zero's?
Now network booting for the mother Pi3? 5 connected Pi's - no SD's

Rats, HDMI is same side as USB, else 5 HDMI outputs no SD, USB extension cables?
Need to get a few more Zero's, only got 4 but two are Zero W which I want to save for bots.
Time to start camping out on disty webshops.

That sounds like it could be interesting. I'm working on a cluster of four Zeroes, one of which has a HubPiWi attached to provide USB connections for the other three and WiFi for connection to the rest of the world.

I removed the USB sockets from the HubPiWi and ran thin cables for the data through the GPIO connector holes of each Zero and soldered them the the PP pads by the USB. I also soldered a couple of bits of solid core wire to connect together all the Pin 2 and also the Pin 6 of each Pi, so all I need to do is provide power to one of them. I just need to work on the software and networking now.

at least two reads of config.txt seems to make sense bootcode.bin and start.elf are different firmware and looks like each are reading it, the other reads who knows perhaps different sub programs in start.elf or one read is looking for one thing (what speed are we clocking to), and another is looking for something else (do they want to start the arm in 32 bit mode or 64).
Could be separate developers wrote different parts of that firmware.

The rpiboot software seems to be initially just for USB msd.
But it does seem to be getting usable as a way to load an OS into ram and run it.
Not sure it it will ever be the way to run full Raspbian on a Zero

Got a Cluster hat which takes four Zero's, one way to get 100 GPIO on a Pi2/3
Rpiboot could be extended into a tool for using Zero's as programmable USB gadgets for any Pi/PC/OS.

bootcode. bin and start.elf change too, newer ones are better, whoops 7 days ago, newer still .
If we use Zero's as GPIO extenders we probably don't need the jpeg stuff which means using the cutdown versions - start_cd.elf etc. This means even faster booting as these are much smaller.

The possibilities of using Zero's for high speed ADC/logic analyzers, USB LCDs, HDMI displays, even parallel image sensors etc Pi Camera is a bit more complex as it requires the start_x.elf versions plus the firmware. But raspistill/vid etc do run on piCore if a way can be figured out to load the required extensions.

Maybe Buildroot could be used to make a OS with everything loaded?
Raw-CSI is getting workable and latest firmware seems to be splitting out the ISP pipeline, baremetal camera one day?

At the moment Ultibo and other baremetal tools seem to be good enough to make a kernel.bin that can do GPIO stuff.
Just the four files are needed, bootcode.bin, start.elf, fixup.dat, kernel.bin. fixup.dat not really needed if you can live with only 256MB of ram, but it is not very big anyway. config.txt if any tweaking is needed?

Not sure if you're still interested in this but I've got this working on the Cluster HAT and it seems to be quite stable (with over 2000 successful boots) and should also works with just a USB cable connecting the Pi Zero to a Pi 3 for example.

I have a guide and an image for download on https://goo.gl/E3RBSR and the script I used to create it is linked at the bottom of the page.

It uses a modified rpiboot (https://github.com/burtyb/usbboot) which supports overlay boot directories (based on USB path) so I can send a different cmdline.txt/etc. to each Pi Zero so they NFS mount a different root directory.